First thing to do is to disconnect the hard drive and don't use it anymore until you know what could be wrong or when you have the right software. The longer it stays connected, the bigger the change is you lose data. (not completely true, but for the sake of simplicity).
I'm gonna go ahead and say your data is corrupt.
Look for recovery software on the internet. There are multiple options to get your data back and most of them have a big pricetag. There is a program, though, that is free and it's called Recuva. It's by piriform. Try if that program can find your data back. If not, think about the value of the data and how much money you are willing to spent, because this could get very nasty.. Good Luck.